The present invention relates to a thyristor of the overvoltage self-protection type which can be safely turned on without being damaged, even when a forward overvoltage exceeding a breakdown voltage is applied between the anode and cathode of the thyristor.
In a case where a forward overvoltage exceeding a breakdown voltage is applied between the anode and cathode of a thyristor, the thyristor is turned on by the overvoltage at an unfavorable portion thereof which is different from a portion where the thyristor is normally turned on, and may as a result become thermally damaged by an excessive current flowing through the unfavorable portion.
A method of protecting a thyristor against the damage resulting from an overvoltage is to connect an external protection circuit to the thyristor. This method, however, is disadvantageous in that the number of parts used is increased and thus a reduction in reliability and an increase in manufacturing cost thus become unavoidable. Another method of protecting a thyristor against the damage due to an overvoltage is to cause the thyristor itself to have a protective function against the over-voltage. A thyristor having a self-protective function against an overvoltage is called "thyristor of the over-voltage self-protection type".
A typical one of conventional thyristors of the overvoltage self-protection type (referred to as Japanese Laid-Open Patent Application No. JP-A-60-42864) is shown in FIG. 8. Referring to FIG. 8, a semiconductor substrate 1 includes a P-emitter layer 11, an N-base layer 12 contiguous to the P-emitter layer 11 for forming a first PN junction J.sub.1, a P-base layer 13 contiguous to the N-base layer 12 for forming a second PN junction J.sub.2, a main N-emitter layer 14 contiguous to a portion of the P-base layer 13 for forming a third PN junction J.sub.3, and an auxiliary N-emitter layer 15 contiguous to the P-base layer 13 and spaced apart from the main N-emitter layer 14 for forming a fourth PN junction J.sub.4. Further, a circular recess 16 is formed in the exposed surface of the P-base layer 13 at a central portion of the exposed surface. The auxiliary N-emitter layer 15 is formed around the recess 16 coaxially therewith, and the main N-emitter layer 14 is arranged on the outside of the auxiliary N-emitter layer 15 coaxially therewith. A P.sup.+ -surface layer 17 which is higher in impurity concentration than the P-base layer 13, is formed on the surface of the recess 16. Further, in FIG. 8, reference numeral 2 designates an anode kept in ohmic contact with the exposed surface of the P-emitter layer 11, 3 a cathode kept in ohmic contact with the exposed surface of the main N-emitter layer 14, 4 an auxiliary electrode kept in contact with the auxiliary N-emitter layer 15 and the P-base layer 13, and 5 an annular gate electrode provided on the P-base layer 13 between the recess 16 and the auxiliary N-emitter layer 15.
When a forward voltage is applied between the anode 2 and the cathode 3 of a thyristor having the above structure in such a manner that the anode 2 is positive with respect to the cathode 3, a reverse bias voltage is applied across the second PN junction J.sub.2, and thus the thyristor is put in a forward blocking state. At this time, a depletion layer is formed in the N-base layer 12 and the P-base layer 13. The limits of the depletion layer are indicated by broken lines in FIG. 8. The depletion layer is extended in the P-base layer 13 so as to go beyond the bottom of the recess 16, but is extended in the P.sup.+ -surface layer 17 at the recess 16. As a result, a multiplication factor obtained in the vicinity of the recess 16 becomes greater than that obtained in the remaining portion, and thus an avalanche breakdown region is locally formed. When the avalanche voltage breakdown region is initially made conductive by an overvoltage, the initial turn-on current thus produced flows as indicated by lines with an arrow. When viewed from the auxiliary N-emitter layer 15 and the main N-emitter layer 14, the initial turn-on current plays the same role as played by a gate triggering current from the gate electrode 5. Thus, when a thyristor having the structure of FIG. 8 is applied with an overvoltage, the thyristor is turned in accordance with a mechanism similar to a normal turn-on mechanism. That is, a thyristor of the overvoltage self-protection type can be realized by the structure of FIG. 8.
However, the thyristor of the overvoltage self-protection type shown in FIG. 8 has the following drawbacks. That is, the initial turn-on current generated within the P.sup.+ -layer 17 travels a long distance to reach the auxiliary N-emitter layer 15 or main N-emitter layer 14, and hence is decreased by carrier recombination. Further, the initial turn-on current is spread in the direction of the thickness of the P-base layer 13, and thus a current useful for forward biasing the fourth PN junction between the P-base layer 13 and the auxiliary N-emitter layer 15 is decreased. As a result, an initially conductive region formed by the initial turn-on current is very small and hence the so-called hot spot is readily formed. Thus, there is a fear of the thermal damage to the thyristor.